In recent years, in view of global environment protection, it has been conducted in the field of automobiles to develop technology for more improving specific fuel consumption and more purifying exhaust gas. As a part of such development, technology development on hybrid vehicles and electric vehicles have been promoted. With respect to the development of these technologies, practicable electric double layer capacitors have attracted attention for the use of driving system power assist or energy regeneration. Principle of the electric double layer capacitors is based on accumulation of charge between electric double layers formed at the interface region between a polarizable electrode and an electrolytic solution, advantage of which is that, in comparison with secondary batteries such as lead storage battery and nickel hydrogen secondary battery, they permit rapid charge and discharge at a large current. As a material for the polarizable electrode, activated carbon is commonly used due to its large interface and excellent conductivity. Electric double layer capacitors demanded for the use of hybrid vehicles and electric vehicles are required to have not only an electrostatic capacity (F) but a high power (W) density permitting repeated charge-and-discharge at a large current of 100 A order as well, and many technical problems still remain unsolved. Thus, in order to increase the power density, there have been proposed such electrode-manufacturing techniques as that thickness of porous electrodes mainly comprising activated carbon, thickness of current collectors and thickness of separators are made adequate (Japanese Patent Laid-Open No. 317332/1999) and that a composite electrode of activated carbon impregnated with aluminum is made as a polarizable electrode (International Patent Laid-Open No. 509560/1998).
On the other hand, it has been disclosed to use a carbonaceous material represented by activated carbon as an electrode material for electric double layer capacitors in Japanese Patent Publication No. 15138/1985, Japanese Patent Laid-Open Nos. 187614/88, 321620/1989 and 180013/1991, Japanese Patent Publication Nos. 56827/1994, 44407/1992 and 70770/1992.
Japanese Patent Laid-Open No. 321620/1989 discloses to use powdery activated carbon having been heat-treated at 1000° C. as an electrode material for capacitors, and Japanese Patent Laid-Open No. 180013/1991 discloses that electrostatic capacity of an electric double layer capacitor can be improved by adjusting amount of oxygen contained in activated carbon to from 25 to 35% by weight. In both publications, however, kinds and physical properties of the activated carbon are not disclosed at all.
In Japanese Patent Publication No. 70770/1992, it is disclosed that capacitors using activated carbon having an average pore diameter of 1.5 nm (15 Å) or more as an electrode material for the capacitor show excellent temperature properties and, in Japanese Patent Laid-Open No. 187614/1988, it is disclosed that powdery activated carbon showing a high electrostatic capacity has a specific surface area of 1800 to 3500 m2/g and an average pore diameter of 0.5 to 1.5 nm (5 to 15 Å), with the ratio of pore volume of pores having a diameter of 2.0 nm (20 Å) or more to the total pore volume being 20 to 40%. In these publications, however, it is not disclosed to select a specific range of 1.95 nm (19.5 Å) to 2.20 nm (22 Å) as to average pore diameter and select a range of 0.05 cm3/g to 0.15 cm3/g as to pore volume of pores having a pore diameter of 5.0 nm (50 Å) to 30.0 nm (300 Å), as are specified in the invention.
With respect to fibrous carbons, Japanese Patent Publication No. 15138/1985 discloses that fibrous carbon obtained by carbonizing phenolic fibers and conducting activation treatment is suited as an electrode material for capacitors and, in Japanese Patent Publication No. 56827/1994, it is disclosed that carbon fibers or carbon powder having a surface acidic functional group at a concentration of 1.0 meq./g or less is used as an electrode material for capacitors, that those with a specific surface area of 500 m2/g or more are preferred, and that phenol resin-based activated carbon fibers are specifically used. In addition, Japanese Patent Publication No. 44407/1992 discloses to use a product obtained by carbonizing and activating a specific phenol resin foam as an electrode material for capacitors.
However, although activated carbons obtained by carbonizing and activation-treating phenol resin-based materials as disclosed in Japanese Patent Publication Nos. 15138/1985, 56827/1994 and 44407/1992 have about the same specific area as that in the invention but, since the phenol resin-based carbonization products to be activated are of generally amorphous carbon, most of pores formed in the activated carbon obtained by activating the carbonization products have a pore diameter of 15 Å or less. Hence, as will be shown in Comparative Example 4 to be given hereinafter, the activated carbon usually has an average pore diameter of 18 Å or less which is smaller than the average pore diameter in the invention, and have substantially no pore distribution in a comparatively large pore region of 5.0 to 30.0 nm.
Besides, since electrode materials for electric double layer capacitors disclosed in these publications are intended to increase electrostatic capacity per unit weight or per unit volume (F/g or F/cm3), discharge current density per unit area of the electrode shown in the charging and discharging test conducted in the specific examples is at a considerably low level of about 0.1 to about 2 mA/cm2, and effects as electrode material at a high current density of, for example, 10 mA/cm2 or more are not confirmed at all, thus these publications not disclosing activated carbons suited for the high power density uses at all. Activated carbon-fiber woven cloth prepared from phenol resins, disclosed in Japanese Patent Publication No. 15138/1985, has an advantage that it has a smaller electric resistance than electrodes obtained by molding or coating powdery activated carbon and, in this sense, it is preferred because a high power density is expected. However, it has the defect that capacity per volume is small though output per weight is large due to the small bulk density of the electrode made of it.
Accordingly, the electric double layer capacitors using as electrode material the activated carbons disclosed in these publications are not necessarily said to give a large output density per unit volume, though they show an increased energy density due to a comparatively large electrostatic capacity thereof.
Further, advantages required for the electric double layer capacitors are that number of usable charge-and-discharge cycles be large and that they have a high durability such as that they undergo a smaller decrease in capacity in a continuously voltage-applying test, as compared with those of the conventional secondary batteries. The electric double layer capacitor using, as an electrode material, activated carbon containing a large amount of oxygen as disclosed in the foregoing Japanese Patent Laid-Open No. 180013/1991 suffers a serious decrease in capacity due to an increase in resistance or the like upon charging and discharging, thus involving a serious problem as to durability.
With electrode materials using the known activated carbons described hereinbefore, it is possible to decrease inner electrical resistance of the electrode and increase the power density to some extent by improving the method for manufacturing the electrodes, but an activated carbon which gives electric double layer capacitors showing a high power density and an excellent durability still remains unfound.
An object of the present invention is to provide an activated carbon suited for producing an electric double layer capacitor which shows a large output density per unit volume and which, even when subjected to repeated charge-and-discharge cycles at a large electric current or even when a constant voltage is continuously applied thereto for a long period of time, undergoes only a slight decrease in power density.